Easiest way to remove all lines that don't start with a given substring from a text file?

Question

Image processing

I am not strong in programming, but I assume that this is not a difficult task. Perhaps there are some user functions in some text editor that will solve my problems.
Question: can you share an idea how to solve the problem?

Answer 1

Open Excel, take your file, paste it into the table (so that each line is on a separate line of the table)
Then, using the “sort and filter” button, set it so that only the lines you need remain.
After that, select them all and copy them to a new file.
Actually that's all

Answer 2

cat file | sed '/regexp/d' > file2

or

cat file | grep -v -e regexp > file2

Answer 3

grep -v ^string file.txt > file.txt

Answer 4

apparently I'm not just not strong, but generally not aware. Where can such a string be entered?

Answer 5

What programming environment are we talking about?